An IP network here refers to a switching and transmission network, in which communication signals are switched or transmitted on the basis of an internet protocol.
Switching network configurations of the type mentioned above will in future supplement or even supersede conventional communication networks, such as time-multiplex switching networks. In the transition devices between the switching centers of the first communication network and the second communication network according to the respective network conditions. If the first communication network is a time-multiplex switching network, such as the communication network deployed commercially under the name EWSD, and if the second communication network is for example a packet switching network, such as an IP network, it is necessary to effect the conversion from time-multiplex signals to packet signals and vice-versa from packet signals to time-multiplex signals in the transition devices between the switching centers of the first communication network and the second communication network respectively. This means that in the event of a communication connection between a calling subscriber station linked to a switching center of the first communication network and a subscriber station to be called also linked to a switching center of the first communication network, in particular another switching center of the relevant first communication network, a dual signal conversion is required based on the signal conversions considered above. This requires a relatively high conversion capacity and therefore computer capacity in the transition devices provided between the relevant switching centers of the first communication network and the second communication network, the transition devices generally being computer-controlled, in order to achieve the fastest possible conversions of the communication signals. Such fast conversions of communication signals are necessary so that the delays in the forwarding of communication signals resulting from the relevant conversions are not perceived as disruptive and detrimental to the quality of the respective communication connection.
In addition to the problem highlighted above, a further, in some cases more serious, problem also occurs in the event of call forwarding or call redirection in respect of a subscriber station to be called. With such call redirection a communication connection is first set up in the conventional manner to the switching center, at which the subscriber station to be called was hitherto available, and a redirection communication connection is then set up from this switching center to the switching center at which the relevant subscriber station to be called is now available. However, given the switching system configuration considered above, this means that in addition to the conversion processes considered above, further conversion processes also have to be implemented in respect of the communication signals to be transmitted in each instance between the calling subscriber station and the subscriber station to be called. However this is associated with marked delay times and therefore also a deterioration in quality during transmission of the respective communication signals between the calling subscriber station and the subscriber station to be called.
These delay times result primarily from packet assembly and disassembly processes due to the transitions between the switching networks. Given a typical packet assembly or disassembly time of between 10 and 30 ms and a delay threshold value of around 100 ms due to the sensitivity of human hearing in respect of sound delays, an n-fold conversion of communication signals, by which the resulting overall delay time due to conversion exceeds the said threshold value, is no longer perceived as tolerable.
With regard to the delay threshold value just mentioned, it should also be taken into account that sections subject to a high level of delay may be included during the course of the respective communication connection, such as one or a plurality of mobile radio links or a satellite link.
Several documents are known from the state of the art to resolve the problem:
In EP 0 905 959 A2, “A packet switched network telephone system”, dated 31 Mar. 1999 a telephone system is also disclosed, with which a connection can be switched between two telephones via a packet switching network by exchanging network addresses. The system also allows a number of additional services, such as callback, call forwarding, call waiting and many more. In the case of call forwarding for example a source switching node, to which the calling telephone is linked, attempts to set up a connection to a called telephone, unaware that call forwarding is stored for this called telephone. As a result the new call number is sent from the destination switching node to which the telephone to be called is linked, to the source switching node, which then sets up a direct connection to the new call destination.
Also in U.S. Pat. No. 4,764,919, “Virtual PBX call processing method”, dated 16 Aug. 1988 a group of users in a packet data network is given access to a virtual PBX functionality. The user stations thereby exchange signaling information via virtual connections and process this information, to implement call control. Services such as call forwarding or automatic callback are thereby implemented without the assistance of the packet data network. With call forwarding a source user station attempts to set up a connection to a destination user station, unaware that call forwarding is stored for this user station. As a result the new call address is sent from the destination user station to the source user station, which then sets up a direct connection to the new user station.
In this context WO 00/31933, “Voice over data telecommunications network architecture”, dated Feb. 6, 2000 is also of interest, describing a system and a method for voice and data transmission via a packet switching network, which exists in parallel to a conventional PSTN network and communicates with this. The system allows the packet switching of voice and data connections from and to every local network operator, a user terminal device or a data network.
In U.S. Pat. No. 6,292,553, “Method and apparatus for IP network call delivery”, dated 18 Sep. 2001 a method is also disclosed, with which a caller can only set up a connection to another subscriber via an IP network by dialing the latter's number. On receipt of a request for a call to be set up, a network accesses a database, which contains control commands for route control. If an appropriate entry is found in the database for what is known as a call attribute, the call is routed according to the control commands, if necessary also via the IP network, if this is specified.
U.S. Pat. No. 6,205,135, “Access platform for internet based telephony”, dated 20 Mar. 2001 also discloses an alternative access platform, which is linked to the internet, a local network operator and other communication devices. A user thereby sets up an internet connection for example or even a POTS connection to the access platform. The connection is routed according to a profile assigned to a user, independently of this.
Finally, WO 97/28628, “Hybrid network for real-time phone-to-phone communications”, dated Jul. 8, 1997 a method and a system are disclosed that allow a subscriber in a conventional telephone network to make long-distance calls via a packet switching data network, to save on call charge costs. To make a long-distance call the sub-scriber only has to specify the address of a source server and the destination call number. The source server then determines the destination server to which the connection should be set up. The destination server then dials the destination call number and sets up the connection between the calling parties.